The Weight In Your Hand (equals/does not equal) The Force On Your Body

Whenever I prepare a new client for beginning an exercise program, I start with an explanation of how important it is to have correct posture and form.
Unfortunately, the majority of professionals and exercise enthusiasts maintain the perspective that exercise is as simple as picking up a weight or sitting on a machine and moving an object from point A to point B until it generates a good sweat and some soreness the next day. " No Pain- No Gain "? Pain is the result of physical damage, the body’s way of telling you something is wrong. Working through pain results in producing muscle memory based on compensating for previous damage done.
Exercise is a force applied to a specific pattern. Force is basically a push or pull exerted by one object onto another object like gravity, cable or tubing pulling or pushing on a limb. Force does not exactly equate to the amount of actual resistance in an exercise. Your joints move in an arched motion and force is typically directed in a straight line. When you match those two together you don't get a constant resistance and therefore a constantly changing stress.
For example, if you hold a 5lb dumbbell in your hand, resting at your side, that dumbbell is only 5lbs of resistance in your hand. Since the force is going straight down, and your arm is in line with the line of force, the resistance of the 5lb weight to your shoulder is 0. But if you move the 5lbs out and to the side in line with your shoulders, directly against the force of gravity, the actual resistance at this distance increases dramatically.
The fact is, force is calculated by the formula F=MxD where the actual force is equal to the mass times the distance from the point of rotation or joint doing the work. Each muscle has a specific effective range of motion and capacity for stress. Exceeding either will result in muscle and/or joint damage.
Understanding the limits of range of motion, joint limitations, and effective resistance provides the only true method for maximizing physical performance while minimizing the potential for injury.
This is one small part of explaining how physics comes into play when looking to manipulate different types of exercise safely and effectively. Once you understand where the line of force is coming from, you can choose how it challenges your muscles.